When handling volatile liquids such as hydrocarbons including gasoline and kerosene, air-volatile liquid vapor mixtures are readily produced. The venting of such air-vapor mixtures directly into the atmosphere would result in significant pollution of the environment and a fire or explosion hazard. Accordingly, existing environmental regulations require the control of such emissions.
As a consequence, a number of processes and apparatus have been developed and utilized to recover volatile liquids from air-volatile liquid vapor mixtures. Generally, the removed volatile liquids are liquified and recombined with the volatile liquid from which they were vaporized thereby making the recovery process more economical.
The initial vapor recovery systems utilized in the United States in the late 1920's and early 1930's incorporated a process combining compression and condensation. Such systems were originally only utilized on gasoline storage tanks. It wasn't until the 1950's that local air pollution regulations began to be adopted forcing the installation of vapor recovery systems at truck loading terminals. Shortly thereafter, the "clean air" legislation activity of the 1960's, which culminated in the Clean Air Act of 1968, further focused nationwide attention on the gasoline vapor recovery problem. As a result a lean oil/absorption system was developed. This system dominated the marketplace for a short time.
Subsequently, in the late 1960's and early 1970's cryogenic refrigeration systems began gaining market acceptance (note, for example, U.S. Pat. No. 3,266,262 to Moragne). While reliable, cryogenic systems suffer from a number of shortcomings including high horsepower requirements. Further, such systems require relatively rigorous and expensive maintenance to function properly. Mechanical refrigeration systems also have practical limits with respect to the amount of cold that may be delivered, accordingly, the efficiency and capacity of such systems is limited. In contrast, liquid nitrogen cooling systems provide more cooling than is required and are prohibitively expensive to operate for this type of application.
As a result of these shortcomings of cryogenic refrigeration systems, alternative technology was sought and adsorption/absorption vapor recovery systems were more recently developed. One such system is disclosed in, for example, U.S. Pat. Nos. 4,066,423 to McGill et al. Such systems utilize a bed of solid adsorbent selected, for example, from silica gel, certain forms of porous mineral such as alumina and magnesia, and most preferably activated charcoal. These adsorbents have an affinity for volatile hydrocarbon liquids. Thus, as the air-hydrocarbon vapor mixture is passed through the bed, a major portion of the hydrocarbons contained in the mixture are adsorbed on the bed. The resulting residue gas stream comprising substantially hydrocarbon-free air is well within regulated allowable emission levels and is exhausted into the environment.
It should be appreciated, however, that the adsorbent is only capable of adsorbing a certain amount of hydrocarbons before reaching capacity and becoming ineffective. Accordingly, the bed must be periodically regenerated to restore the carbon to a level where it will effectively adsorb hydrocarbons again. This regeneration of the adsorbent is a two step process.
The first step requires a reduction in the total pressure by pulling a vacuum on the bed that removes the largest amount of hydrocarbons. The second step is the addition of a purge air stream that passes through the bed. The purge air polishes the bed so as to remove substantially all of the previously adsorbed hydrocarbons. These hydrocarbons are then pumped to an absorber tower wherein lean oil or other nonvolatile liquid solvent is provided in a countercurrent flow relative to the hydrocarbon rich air-hydrocarbon mixture being pumped from the bed. The liquid solvent condenses and removes the vast majority of the hydrocarbons from that mixture and the residue gas stream from the absorber tower is recycled to a second bed of adsorbent while the first bed completes regeneration.
Up to the present date, cryogenic vapor recovery systems and adsorption/absorption vapor recovery systems have largely been independent technologies offered by different companies competing for a share of the vapor recovery system market. Little has been done to combine these technologies. Further, those efforts to combine the technologies have not achieved the most beneficial result.
For example, in U.S. Pat. No. 4,343,629 to Dinsmore et al, a cooling medium is circulated through heat transfer coils in the adsorbent beds. This is done to prevent the beds from overheating due to side exothermic reactions of hydrocarbons and/or impurities contained in the air-hydrocarbon vapor mixture with air and/or the solid adsorbent. While such an approach improves the efficiency of adsorption of hydrocarbons by the bed through the provision of lower operating temperatures, this approach fails to address other important issues. For example, high levels of moisture and oxygenates such as alcohol in the air-hydrocarbon vapor mixture and heavy hydrocarbons from distillates reach and contact the adsorbent bed adversely affecting adsorption efficiencies and shortening the service life of the bed.
In U.S. Pat. 4,480,393 to Flink et al, refrigeration condensation is utilized to recover the liquid hydrocarbons during regeneration of the bed. Once again, however, it should be appreciated that this approach allows the full level of moisture and oxygenates in the original air-hydrocarbon vapor mixture as well as heavy hydrocarbons to reach and contact the bed. Thus, as discussed above, the adsorption efficiency and the functional life of the bed are both significantly reduced to the detriment of the operator. Accordingly, it should be appreciated that a need is identified for an improved vapor recovery system that takes full advantage of a combination of cryogenic and adsorbent/absorbent vapor recovery system technologies.